1. Field of the Invention
This invention generally relates to an apparatus for performing a highly efficient coding or an image standards conversion, which is for use in a system for recording, transmitting and displaying digital image signals. More particularly, this invention relates to a motion compensation apparatus for performing a motion compensation processing (or movement compensation processing) on dynamic image signals with a high precision to one pixel or pel, namely, to the interval or distance (hereunder sometimes referred to simply as a pixel interval or distance) between two adjoining pixels.
2. Description of the Related Art
A typical method for performing a highly efficient coding of dynamic image signals is an interframe prediction coding method, by which a frame to be coded is predicted from a previous frame already coded and only a coding of prediction error is performed. In case of employing such an interframe prediction coding method, what is called a motion-compensated interframe prediction for effecting the prediction by changing an image in accordance with the motion of a moving object has come to be generally performed.
Further, it has become known that when an interpolation of scanning lines or frames is performed, deterioration of resolution or the like can be made to be small in case where a motion compensation is effected in an image standards converter for converting interlace signals into non-interlace signals or converting an image signal of a certain frame frequency into another image signal of a different frame frequency.
In case where the accuracy of a motion vector is equal to or coarser than the pixel interval, such a motion compensation operation is merely to shift the position of a pixel. However, in case of performing a motion compensation with high accuracy finer than the pixel interval, a motion compensation signal is generated by effecting a resampling processing.
Further, such a processing can be more appropriately done by performing a prediction and an interpolation by using a plurality of frames.
Hereinafter, a conventional motion compensation apparatus will be described.
Referring to FIG. 4, there is shown the configuration of an example of a conventional motion compensation apparatus which performs a motion compensation on a reference image to be processed, namely, a reference field with high precision from two fields (hereunder sometimes referred to as precedent and subsequent fields) respectively precedent and subsequent to the reference field.
A precedent field signal representing a precedent field is inputted from a precedent-field-signal input terminal 3 to both of a motion vector (MV) estimating device 5 and a pixel shift device 7, and on the other hand a subsequent field signal representing a subsequent field is inputted from a subsequent-field-signal input terminal 4 to both of another MV estimating device 6 and a pixel shift device 8. Moreover, a reference field signal representing the reference field is inputted from a reference-image input terminal 2 to both of the MV estimating devices 5 and 6.
In the MV estimating device 5, motion vectors representing the displacement of a moving object, which is shown in a dynamic image (namely, the reference image), between the precedent and reference fields are estimated from the precedent field signal and the reference field signal. Similarly, in the MV estimating device 6, motion vectors representing the displacement of the moving object between the reference and subsequent fields are estimated from the reference field signal and the subsequent field signal. Such motion vectors are estimated by performing a pattern matching process (namely, what is called a block matching process) on each block, which consists of 16.times.16 pixels, of the reference field, namely, by evaluating predetermined measure of the prediction error corresponding to a block of the reference field and blocks of the precedent or subsequent field, which are indicated by what is called trial motion vectors, and then determining one of the trial motion vectors corresponding to a minimum prediction error as the motion vector corresponding to the block of the reference field and effecting such a process on each of the other blocks of the reference field.
Thereafter, signals representing the motion vectors estimated by the MV estimating device 5 (hereunder sometimes referred to as the precedent motion vectors) are outputted therefrom through a precedent MV output terminal 11 to another circuit and moreover are supplied to the pixel shift device 7 and a micro-shift device 17. Similarly, signals representing the motion vectors estimated by the MV estimating device 6 (hereunder sometimes referred to as the subsequent motion vectors) are outputted therefrom through a subsequent MV output terminal 12 to another circuit and moreover are supplied to the pixel shift device 8 and a micro-shift device 18.
Incidentally, in a coding system, it is necessary for performing a decoding processing later to output the motion vectors. However, in case of an image conversion system, it is not necessary for the motion compensation apparatus to output the motion vectors because the image conversion system has only to obtain motion-compensated image signals. In contrast, a decoding system does not estimate motion vectors but receives motion vectors from a coding system.
In the pixel shift device 7, a pixel represented by the precedent field signal is shifted on the basis of the precedent motion vector with precision that is equal to the pixel interval. Then, a signal representing the shifted pixel is fed to the micro-shift device 17. Subsequently, to perform a resampling processing, the micro-shift device 17 multiplies data representing each of such pixels by a coefficient corresponding to a motion represented with accuracy to further the pixel interval and further adds results of such multiplications up. Namely, among values equal to or less than a pixel distance, which values are respectively indicated by information on the motion vectors, part of such information represented with precision, which is equal to or finer than the pixel interval, is used by the pixel shift device 7. Further, the remaining part of such information represented with precision, which is finer than the pixel interval, is used by the micro-shift device 17. The thus motion-compensated precedent-field signal is supplied to an adder 14.
Similarly, in the pixel shift device 8, a pixel represented by the subsequent field signal is shifted on the basis of the subsequent motion vector with precision that is equal to the pixel interval. Subsequently, the micro-shift device 18 multiplies data representing each of the shifted pixels is multiplied by a coefficient corresponding to a motion represented with accuracy to the pixel interval and further adds results of such multiplications up. Further, the micro-shift device 18 supplies signals representing results of such operations as the thus motion-compensated subsequent-field signal to the adder 14. Then, the adder 14 adds both of data respectively represented by the thus motion-compensated precedent-field and subsequent-field signals and outputs a signal representing a result of this addition as a motion-compensated signal through a motion-compensated-signal output terminal 15 to another circuit.
Here, note that in the foregoing description, the processing has been described as performed on each field of an interlace signal, but can be performed on each frame thereof similarly.
Further, in such an interlace signal generally used in television broadcasting, each field signal is obtained by "thinning out" a frame signal and therefore contains many aliasing frequency-components (hereunder sometimes referred to as aliasing components). Moreover, even in case of a non-interlace signal, each field signal includes aliasing components if the diameter of an electron beam of a television camera is smaller than the interval between adjoining scanning lines.
When a high-precision motion compensation is performed on such a field signal in the conventional motion compensation apparatus, an image generated by performing a resampling processing is not correct due to the aliasing components thereof. Thus the conventional motion compensation apparatus has a defect in that appropriate prediction and interpolation cannot be achieved.
The present invention is accomplished to eliminate such a defect of the conventional motion compensation apparatus.
It is, accordingly, an object of the present invention to provide a motion compensation apparatus which can obtain more suitably motion-compensated signals as a result of generating image signals, the density of which is higher that of field signals, by adding up the field signals when high-precision motion-compensated signals are obtained from a plurality of fields, and then performing a resampling processing on the image signals by using resampling coefficients corresponding thereto.